Use astropy to get package data everywhere

[wtbarnes]

Instead of constructing paths manually, as is done here,

xrtpy/xrtpy/response/effective_area.py

Lines 41 to 43 in d3e994c

	_ccd_contam_filename = (
	Path(__file__).parent.absolute() / "data" / "xrt_contam_on_ccd.geny"
	)

it would be better to use the get_pkg_data_filename function to do this (see #99).

Unfortunately, this is complicated by the fact that these bits of code are currently run at import time which results in an import error because the xrtpy.response namespace cannot be resolved immediately at import time.

Instead, I would recommend that this code to read these files be moved into the objects themselves, unless doing so would create a lot of overhead in reading these files.